1. Field of the Invention
The present invention relates to a plane lighting unit used for a back light of a flat panel display, and more particularly to a lighting unit in which a fluorescent discharge tube is provided on a side surface of a light guide board.
2. Description of the Background Art
In display units (monitors) of notebooks, mobile TV sets, car navigation systems and the like, liquid crystal displays which have characteristic features of light weight, slim line and low power consumption are widely used. Most of the liquid crystal displays each contain a lighting unit to achieve high-luminance display, and the construction in which the lighting unit emits illumination light from the rear side of its display element has become the mainstream.
The lighting unit using a fluorescent discharge tube, which is used for a liquid crystal display, is roughly classified into two major types, i.e., an edgelight type in which a light guide board is provided on an emission surface and a fluorescent discharge tube is provided on a side surface of the light guide board and a direct type in which a plurality of fluorescent discharge tubes are provided on an emission surface. Among these two major types, the edgelight type which has an advantage of being slim and light is used for the liquid crystal displays of notebooks, mobile TV sets, car navigation systems and the like. Japanese Patent Application Laid Open Gazette No. 8-6019 (Patent Document 1) shows an invention to improve the luminance of the edgelight type unit.
When a high voltage is applied between electrodes provided at both end portions of the fluorescent discharge tube, electrical discharge is caused. Mercury sealed inside the tube is vaporized and excited by the electrical discharge. A fluorescent film formed on an inner wall of the fluorescent tube is stimulated by ultraviolet rays generated by the excitation, to thereby emit visible light. Since a holder which protects the end portion of the fluorescent discharge tube, however, radiates heat of the fluorescent discharge tube which is in contact with it to the outside of itself, most of mercury vapor is cooled and condensed at the end portion of the fluorescent discharge tube. If the electrical discharge is caused in such a state, the mercury condensed at the end portion is taken in a sputtering material or the like released from the electrode provided at the end portion. As a result, the amount of generated mercury vapor in the entire fluorescent discharge tube becomes low as the discharge time goes by, and the luminance disadvantageously decreases.
To solve this problem, Patent Document 1 shows an invention whose characteristic feature lies in construction in which a projection provided on a holder maintains a clearance between the holder and a frame body supporting the holder with point contact, to provide space therebetween. In this invention, as a material of the holder used is a silicon resin or polycarbonate having lower thermal conductivity.
With such a construction, it is possible to reduce the heat propagated from the fluorescent discharge tube to the frame body through the holder and prevent heat dissipation at the end portion of the fluorescent discharge tube. In this construction, however, heat conduction is surely caused from the fluorescent discharge tube to the holder in accordance with the thermal conductivity of the material of the holder. Further, since the space between the holder and the frame body is not sealed, the heated air moves to the outside space, to be cooled. This results in insufficient thermal insulation.